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Physical Review Letters

Two-Photon Infrared Resonance Can Enhance Coherent Raman Scattering

Andrew J. Traverso, Brett H. Hokr, Zhenhuan Yi, Luqi Yuan, Shoichi Yamaguchi, Marlan O. Scully, Vladislav V. Yakovlev

Published Feb 6, 2018• 30 citations
Publisher

Abstract

In this Letter we present a new technique for attaining efficient low-background coherent Raman scattering where the Raman coherence is mediated by a tunable infrared laser in two-photon resonance with a chosen vibrational transition. In addition to the traditional benefits of conventional coherent Raman schemes, this approach offers a number of advantages including potentially higher emission intensity, reduction of nonresonant four-wave mixing background, preferential excitation of the anti-Stokes field, and simplified phase matching conditions. In particular, this is demonstrated in gaseous methane along the ν_{1} (A_{1}) and ν_{3} (T_{2}) vibrational levels using an infrared field tuned between 1400 and 1600 cm^{-1} and a 532-nm pump field. This approach has broad applications, from coherent light generation to spectroscopic remote sensing and chemically specific imaging in microscopy.

DOI
10.1103/physrevlett.120.063602

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In this Letter we present a new technique for attaining efficient low-background coherent Raman scattering where the Raman coherence is mediated by a tunable infrared laser in two-photon resonance with a chosen vibrational transition. In addition to the traditional benefits of conventional coherent Raman schemes, this approach offers a number of advantages including potentially higher emission intensity, reduction of nonresonant four-wave mixing background, preferential excitation of the anti-Stokes field, and simplified phase matching conditions. In particular, this is demonstrated in gaseous methane along the ν_{1} (A_{1}) and ν_{3} (T_{2}) vibrational levels using an infrared field tuned between 1400 and 1600 cm^{-1} and a 532-nm pump field. This approach has broad applications, from coherent light generation to spectroscopic remote sensing and chemically specific imaging in microscopy.
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We report on an investigation of simultaneous generation of several narrow-bandwidth laser-like coherent emissions from nitrogen molecular ions ( ) produced in intense mid-infrared laser fields. With systematic examinations on the dependences of coherent emissions on gas pressure as well as several laser parameters including laser intensity, polarization and wavelength of the pump laser pulses, we reveal that the multiple coherent emission lines generated in originate from a series of nonlinear processes beginning with four-wave mixing, followed with stimulated Raman scattering. Our analyses further show that the observed nonlinear processes are greatly enhanced at the resonant wavelengths, giving rise to high conversion efficiencies from the infrared pump laser pulses to the coherent emission lines near the transition wavelengths between the different vibrational energy levels of ground X and that of the excited B states.
Stimulated Raman scattering is conventionally employed to probe vibrational dynamics by inducing a coherent oscillation within a molecular ensemble. In contrast, spontaneous Raman scattering is considered as an incoherent process because the scattered field lacks any predictable phase relationship with the incoming laser pulse. It raises questions about the nature of the quantum state of a molecular ensemble following spontaneous Stokes scattering: should it be described as a statistical mixture of vibrationally excited molecules, where collective coherence is absent? Here, we measure time-resolved Stokes--anti-Stokes two-photon coincidences on a molecular liquid and find that a coherent superposition of two collective vibrational modes from two isotopic sub-ensembles is generated under spontaneous Raman scattering. Our results confirm that a spatially extended vibrational coherence emerges when a single collective excitation is spontaneously scattered and detected in a single spatiotemporal mode.
We have investigated the behavior of high-order stimulated Raman scattering in a highly-transient regime. We demonstrate efficient collinear generation of vibrational sidebands in molecular hydrogen and methane using two-color pumping with pulses of duration $\ensuremath{\tau}\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ tuned to a vibrational Raman transition. A Raman spectrum with a large bandwidth was observed, ranging from the IR to the UV. Under some conditions strong pump depletion was observed and up to five anti-Stokes sidebands were observed to have energies exceeding 10% of the transmitted pump pulse energies. A numerical simulation reproduces qualitatively and quantitatively the experimental results and allows us to explain key experimental features. The simulation also confirms that the molecular coherence in the medium is substantially increased by the two-color pumping and allows us to deduce values for the degree of material excitation. The use of this technique looks promising for efficient subfemtosecond pulse generation.